The present invention relates to an image forming apparatus employing electrophotographic technology and particularly to an image forming apparatus which transfers a visible toner image formed on a latent image carrier to a recording medium electrostatially.
In a conventional image forming apparatus, a photoreceptor as a latent image carrier such as a photosensitive drum or a photosensitive belt is rotatably supported to the main body of the image forming apparatus. During the image forming operation, a latent image is formed onto a photosensitive layer of the photoreceptor and, after that, is developed with toner particles to form a visible image. Then, the visible image is transferred to a recording medium. For transferring the visible image, there is a known method of transferring the visible image to the recording medium by using a corona transfer, a transfer roller, a transfer drum, or a transfer belt (hereinafter, referred to as a transfer medium).
Such method is employed in monochrome image forming apparatuses. In addition, for a full-color image forming apparatus having a plurality of photoreceptors and developers, there is a known method transferring a plurality of color images on a transfer belt or transfer drums to a recording medium such as a paper in such a manner that the respective color images are sequentially superposed on each other, and then fixing these images. Moreover, an intermediate transferring type is also known in which color images are sequentially primary-transferred to an intermediate transfer medium and the primary-transferred images are secondary-transferred to a recording medium. Arranged on the photoreceptor and transfer medium used for any of the aforementioned methods is a cleaning mechanism for cleaning residual toner particles remaining on the photoreceptor after the development and the transfer to remove toner particles on the photoreceptor.
To improve the transfer efficiency, it is known in the art to set a peripheral velocity difference between the photoreceptor and the transfer medium. The peripheral velocity difference improves the property of stripping toner particles, thereby improving the transfer efficiency. In case of development using a single-component toner, the toner on the development roller is formed into a thin toner layer as uniformly as possible by a regulating blade in order to impart sufficient triboelectric charge to the toner. In this state, the toner is negatively charged by the surface of the development roller and the surface of the end of the regulating blade.
Especially for full-color image forming apparatuses, the modern trend is toward the use of toner of small particle size, uniform, and high circularity in order to improve the transfer efficiency. However, the use of such a toner reduces the fluidity of toner so that it is hard to cause triboelectric charging by the regulating blade. As a result, it is impossible to give sufficient charge so as to create a charge distribution in the toner. It is inevitable that even toner for negative charge includes positively charged toner particles.
In this case, in an image forming apparatus which forms images by negative charge reversal development, there is a problem that positively charged toner particles adhere to non-image portions of a latent image carrier (photoreceptor) so as to make xe2x80x9cfogxe2x80x9d, thus increasing the actual consumption of toner and also increasing the cleaning load of the photoreceptor. For cleaning residual toner particles after transfer from the photoreceptor, a large container for collecting and receiving the cleaning toner particles is necessary. There is also a problem that the fog increases the consumption of toner, leading to increase in running cost of consumable supplies. If a large amount of superplasticizing agent is added as an external additive to the toner in order to improve the fluidity of the toner for resolving the aforementioned problem, there may be another problem of reducing the fixing property.
For uniformly negatively charging a toner, there is a known method of regulating negatively chargeable toner particles on a toner carrier into a thin layer (the number of stories in the layer is 2 or less) by a toner layer thickness regulating member.
However, the regulation of toner particles into a thin layer reduces the rate of carrying toner particles, thus making the charge of the toner too high. Accordingly, the number of excessively negatively charged toner particles, i.e. over-charged toner particles, should be increased, thus unfortunately lowering the transfer efficiency from the organic photoreceptor to the transfer medium.
If charge is conducted under low temperature and low humidity condition or a color original image of low duty is successively printed on plural sheets of paper, the amount of charge of toner becomes too high, that is, toner particles are excessively charged. As a result, the reproducibility of half tone images should be lowered and/or the saturated printing optical density for printing solid images tends to be lowered.
The aforementioned method of setting a peripheral velocity difference between the photoreceptor and the transfer medium for improving the transfer efficiency of toner has a limit of improving the transfer efficiency under the low temperature and low humidity conditions or the high temperature and high humidity condition.
Another type of image forming apparatus will be described now. The image forming apparatus comprises a photoreceptor as a latent image carrier such as a photosensitive drum or a photosensitive belt which is rotatably supported to the main body of the image forming apparatus. During the image forming operation, a latent image is formed onto a photosensitive layer of the photoreceptor and, after that, is developed with toner particles to form a visible image. Then, the visible image is transferred to an intermediate transfer medium such as a transfer drum or a transfer belt (primary transfer) and then transferred again to a recording medium (secondary transfer).
Such method is generally employed in a full-color image forming apparatus in which color images are sequentially primary-transferred to an intermediate transfer medium and the primary-transferred images are secondary-transferred to a recording medium such as a paper at once. Arranged on the photoreceptor is a cleaning mechanism for residual toner particles remaining on the photoreceptor after the development and the transfer.
As toner used for such an image forming apparatus, dual-component toner composed of a developer and a magnetic carrier is generally known. Though the dual-component toner achieves relatively stable developing, the mixing ratio of the developer and the magnetic carrier is easily varied so that the maintenance for the mixing ratio is required. Accordingly, magnetic single-component toner has been developed. However the magnetic single-component toner has such a problem that clear color images are not obtained due to the opacity of magnetic material thereof. On the other hand, non-magnetic single-component toner has been developed as color toner. For obtaining high-quality record images with the non-magnetic single-component toner, there is a problem how to uniformly charge the toner particles
In order to solve the aforementioned problem of the non-magnetic single-component toner, Japanese Patent Unexamined publication H3-62072 discloses a toner layer thickness regulating member for a developing device. The toner layer thickness regulating member is made of a metal of which work function is low so as to have not only a function controlling the thickness of a toner layer but also a function actively causing triboelectric charging to the toner layer, thereby making charge uniform. This avoid local variation in the developing concentration due to insufficient charge, prevents deterioration of quality of record images, and equalize the thickness of toner layer.
However, the regulation of toner particles into a thin layer reduces the rate of carrying toner particles, thus making the charge of the toner too high. Accordingly, the number of excessively negatively charged toner particles, i.e. over-charged toner particles, should be increased, with the result that toner particles unfortunately strongly adhere to the intermediate transfer medium at the primary transfer portion, thus lowering the transfer efficiency at the secondary transfer portion.
If charge is conducted under low temperature and low humidity condition or a color original image of low duty is successively printed on plural sheets of paper, the amount of charge of toner becomes too high, that is, toner particles are excessively charged. As a result, the reproducibility of half tone images should be lowered and/or the saturated printing optical density for printing solid images tends to be lowered.
In addition, the aforementioned method of setting a peripheral velocity difference between the photoreceptor and the transfer medium for improving the transfer efficiency of toner has a limit of improving the transfer efficiency under the low temperature and low humidity conditions or the high temperature and high humidity condition.
It is a first object of the present invention to provide an image forming apparatus, capable of resolving the aforementioned conventional problems, which comprises a latent image carrier and a developing means for forming a negatively chargeable toner layer, of which the number of stories is 2 or less, on a toner carrier by a toner layer thickness regulating member, wherein an electrostatic latent image on the latent image carrier is developed with the toner into a visible image and said visible image is transferred to a transfer medium, and which prevents the movement of charge from the latent image carrier to the toner, thereby preventing the toner particles from excessively charged and improving the transfer efficiency.
To accomplish the aforementioned object, an image forming apparatus according to claims 1 through 9 comprises: a latent image carrier; and a developing means for forming a negatively chargeable toner layer composed of two stories or less on a toner carrier by a toner layer thickness regulating member, wherein an electrostatic latent image on the latent image carrier is developed with said toner to form a visible image and said visible image is transferred to a transfer medium, and is characterized in that the work function ("PHgr"opc) of the surface of said latent image carrier is set to be larger than the work function ("PHgr"t) of the toner.
According to the invention of claims 1 through 9, the amount of positively charged toner particles which are charged into the opposite polarity can be reduced by regulating toner particles into a thin layer. As a result, the amount of fog toner particles can be reduced. In this case, however, the regulation of toner particles into a thin layer generally makes the toner to be easily negatively charged, thus unfortunately lowering the transfer efficiency after development. However, since the work functions of the toner and the latent image carrier are set to have a relation: "PHgr"opcxe2x89xa7"PHgr"t, no charge moves from the latent image carrier to the toner after the development so that there is no chance that the toner is further negatively charged at the contact portion, thus inhibiting the excessive charge of toner. Since the excessive charge is inhibited, toner particles can be effectively transferred from the latent image carrier to the transfer medium at the primary transfer portion where transfer is conducted according to the constant voltage method. At the secondary transfer portion of the intermediate transfer medium, toner particles can be effectively transferred from the intermediate transfer medium to the receiving medium (such as a paper) because of relatively small triboelectric power of the toner to the intermediate transfer medium. As a result, the amount of residual toner particles remaining on the latent image carrier or the transfer medium becomes little, thereby reducing the cleaning load and the cleaning toner amount. Accordingly, the volume of a container for collecting and receiving the cleaning toner particles can be reduced to be ⅓ or less of the volume of the conventional container. Since the miniaturization of the image forming apparatus itself is achieved and the transfer efficiency is improved, the amount of residual toner on the latent image carrier is reduced, thereby reducing the cleaning load of a cleaning member, leading to the improvement of durability.
It is a second object of the present invention to provide an image forming apparatus, capable of resolving the aforementioned conventional problems, which comprises a latent image carrier and a developing means for forming a negatively chargeable toner layer on a toner carrier by a toner layer thickness regulating member, wherein an electrostatic latent image on the latent image carrier is developed with the toner into a visible image and said visible image is transferred to an intermediate transfer medium, and which reduces the amount of over-charged toner particles to be transferred to the intermediate transfer medium to prevent the toner particles from strongly adhering to the intermediate transfer medium at the primary transfer portion, thereby improving the transfer efficiency at the secondary transfer portion.
To resolve the aforementioned object, an image forming apparatus according to claims 10 through 17 comprises: a latent image carrier; and a developing means for forming a negatively chargeable toner layer on a toner carrier by a toner layer thickness regulating member, wherein an electrostatic latent image on the latent image carrier is developed with said toner to form a visible image and said visible image is transferred to an intermediate transfer medium, and is characterized in that the work function ("PHgr"TM) of the surface of the intermediate transfer medium is set to be larger than the work function ("PHgr"t) of the toner.
According to the invention of claims 10 through 17, in the image forming apparatus comprising a developing means for forming a negatively chargeable toner layer on a toner carrier by a toner layer thickness regulating member, wherein an electrostatic latent image on the latent image carrier is developed with said toner to form a visible image and said visible image is transferred to an intermediate transfer medium, the work function ("PHgr"TM) of the surface of the intermediate transfer medium is set to be larger than the work function ("PHgr"t) of the toner. At the primary transfer portion, since the excessive charge is inhibited by the contact between the intermediate transfer medium and the toner, the amount of toner particles which are excessively charged due to the regulation into a thin toner layer can be reduced. At the secondary transfer portion of the intermediate transfer medium, toner particles can be effectively transferred from the intermediate transfer medium to the receiving medium (such as a paper) because triboelectric power of the toner to the intermediate transfer medium is restrained. As a result, the amount of residual toner particles remaining on the latent image carrier or the transfer medium becomes little, thereby reducing the cleaning load and the cleaning toner amount. Accordingly, the volume of a container for collecting and receiving the cleaning toner particles can be reduced to be xc2xd or less of the volume of the conventional container. Since the miniaturization of the image forming apparatus itself is achieved and the transfer efficiency is improved, the amount of residual toner on the latent image carrier is reduced, thereby reducing the cleaning load of a cleaning member, leading to the improvement of durability.
By regulating toner particles into a thin layer, the toner can be uniformly charged. As a result, the amount of fog toner particles can be reduced by reducing the amount of positively charged toner particles which are charged into the opposite polarity.
Even in case of single-component development with a toner which is regulated into a thin layer for uniformly charged, the work function ("PHgr"TM) of the surface of the intermediate transfer medium is set to be larger than the work function ("PHgr"t) of the toner, thereby preventing the toner from being further negatively charged at the contact portion with the intermediate transfer medium. As a result of this, the transfer efficiency when a constant voltage source is employed can be improved even though the toner is regulated into a thin layer.